Superfluid Dynamics of Quantum Ferrofluids

In state-of-the-art laboratories worldwide, gases of atoms are being cooled down to temperatures less than a millionth of a degree above absolute zero. At this extreme coldness, quantum mechanics takes over; the atoms lose their individual identities and become smeared out into a giant wave of matter. This quantum gas hosts a range of bizarre behaviours, from its capacity to undergo wave-like interference to its embodiment of a superfluid, a fluid with no resistance to motion.

The quantum gas is far from just a scientific curiosity. It represents a clean and pure exemplar of a many-particle quantum system, giving rich insight into the quantum world. Atomic physics techniques empower experimentalists to precisely tune its physical properties and manipulate it in time and space. Due to these facets, quantum gases are being exploited as "emulators" to recreate and understand complicated physical phenomena, from superconductors and turbulence to black holes and the Big Bang. The quantum gas also holds exciting technological prospects. Their exceptional sensitivity to being disturbed is driving their development as ultra-precise sensors, e.g. of gravity, for which they are touted to lead to major advancement in oil and mineral exploration. Meanwhile, their unprecedented quantum control makes these gases candidates for performing quantum gate operations, the basis of the much-lauded quantum computer.

Recent experiments in quantum gases have created a "quantum ferrofluid". Being both a superfluid and a ferrofluid, this novel state lies at the interface of two of our most bizarre fluids. Ferrofluids are liquids dispersed with tiny magnetic iron particles. Just like bar magnets, the particles interact over long-range, prefer to lie with north and south poles being adjacent, and become aligned in an imposed magnetic field. This leads to peculiar patterns and instabilities in the fluid, but, more importantly, enables the flow and physical properties to be controlled via magnetic fields, as exploited in ferrofluid technologies in medicine, information display and sealants.

The quantum sibling of the ferrofluid, the quantum ferrofluid, has been formed from an ultracold quantum gas of magnetic atoms. This gas is being hotly researched to probe its novel properties and potential exploitation. Its magnetic nature extends the above-mentioned capabilities of the quantum gas into new territories, e.g., providing a testbed of quantum magnetism, emulation of systems with long-range interactions, and a sensitivity to magnetic fields which can be exploited in a new generation of magnetic sensors, with potential applications from geological exploration to military detection. Meanwhile, the long-range magnetic interaction between atoms is particularly attractive for quantum computation since it allows the computational operations to be performed at a distance.

The fundamental nature of superfluidity in the quantum ferrofluid remains uncharted, and uncovering it is the core aim of this project. With superfluidity underpinning the transport properties of the system, we will reveal how the quantum ferrofluid moves and flows, swirls and gyrates, and responds to agitation. This is of fundamental interest to our understanding of superfluidity in general, but, more specifically, is of great practical benefit for future manipulation and exploitation of the quantum ferrofluid. The distinctive behaviour of conventional ferrofluids and their virtuous control via magnetic fields is suggestive of a rich plethora of novel superfluid behaviour and a new dimension of control over the superfluid state. The quantum ferrofluid may in turn provide insight into the conventional ferrofluid; being superfluid, with an absence of viscosity, the quantum ferrofluid embodies a simplified version of the ferrofluid from which outstanding problems in ferrofluids can be tackled afresh.

Ultracold quantum gases are exalted for their capacity for precise quantum control and measurement over a macroscopic number of particles. In dipolar quantum gases, this capacity is coupled with the properties of magnetism and long-range interactions, providing a rich platform for the development of quantum technologies. The dipole-dipole interactions offer the possibility of performing quantum gate operations, the basis of quantum information processing, at long-range. Quantum gases in general are being strongly pursued for applications in precision measurement and sensing; prototype experiments have, for example, led to technological advances in the measurement of surface forces and magnetic fields. The magnetic sensitivity of dipolar gases makes them candidates as ultra-precise magnetic sensors, with potential economic impact across a diverse remit including oil and mineral exploration, military detection and biomedical imaging.

An elementary requirement for harnessing and exploiting dipolar quantum gases is an understanding of their fundamental dynamical and transport properties. The dipolar interactions, being anisotropic and long-range, are very different to those which dominate conventional atomic quantum gases, and introduce a host of new phenomena and instabilities. The proposed research seeks to develop a deep understanding of these transport properties. In doing so, it will impact positively upon the current experimental drive to study, harness and ultimately exploit dipolar gases towards these applications. Achievement of these ultimate applications would have significant economic and societal benefits. Key to facilitating the impact of our theoretical findings on this experimental drive is the envisaged establishment of a collaboration with the world's leading experimental dipolar gas group (Stuttgart).

The form of fluidity in these gases - the quantum ferrofluid - lies at the interface of two of mankind's most intriguing and distinctive fluids: the superfluid and the ferrofluid. Superfluids, which are free from viscosity and whose circulation is restricted to be quantized, have proven to embody a simplified and idealized platform for studying classical fluidity, with particular benefit for elucidating complicated dynamics such as turbulence. In a similar vein, we will champion quantum ferrofluids as a prototype of classical ferrofluids, and seek to stimulate the transfer of knowledge and ideas at the interface between these previously disparate fields. Collaboration with a leading UK group in the modelling of classical ferrofluids (Edinburgh) will promote this venture.

Beyond quantum gases and ferrofluids, the proposed research will generate academic impact that spans condensed matter and applied mathematics, due to its direct relevance to the other superfluid systems of liquid Helium and semiconductor cavities, and vortex modelling in applied mathematics. Our development of an innovative vortex-based methodology for modelling the dynamics of the quantum ferrofluid will provide a new and advanced addition to the theoretical toolbox of superfluids and quantum gases. Indeed, to promote the wider use and benefit of this approach the computational codes will be made freely available.

Further economic impact will be made through the training of the post-doctoral researcher and undergraduate student. These individuals will gain expertise across physics, mathematics and computation, develop their communication skills, and network with leading researchers. Specifically, the research associate will also gain training in research management and supervision, while the undergraduate will gain an invaluable taste of high-level research. Finally, in taking place within the newly-formed Joint Quantum Centre Durham-Newcastle, the proposed research will firmly establish the centre's international reputation for state-of-the-art theoretical and experimental activities across quantum gases.